The U.S. Food and Drug Administration has begun trying to find the sources of the tomatoes used in sandwiches at Sheetz convenience stores to determine what might have caused a regional salmonella outbreak.

FDA is moving ahead with the tomato search -- called a "trace-back" -- even though public health officials haven't yet determined whether tomatoes, lettuce or some other food product was the vehicle for the salmonella contamination.

Christopher Snowbeck reported in the Pittsburgh Post-Gazette that State agriculture officials found Monday that one tomato sample from Sheetz was contaminated with salmonella. But it that turned out that it was a different strain of the bacteria than the one blamed in the multistate outbreak, which has sickened roughly 160 people.